CompareTo有问题

Question

我正在尝试实现排序列表.我已经创建了我想要存储在列表中的类,但出于某种原因,当我尝试运行排序方法时,我会抛出异常.幕后的东西似乎传递了我已经实现null的CompareTo方法的引用.

这就是我认为相关的代码.

数据对象定义如下:

namespace PrioritisedRequestQueue
{
public class XactTask : IComparable<XactTask>
{
    public int priority;
    public DateTime submitted;
    public string name;

    public XactTask( int priority, DateTime submitted, string name)
    {
        this.priority = priority;
        this.submitted = submitted;
        this.name = name;
    }

    public int CompareTo(XactTask rhs )
    {
        //EXCEPTION OCCURS HERE AFTER A NULL RHS HAS BEEN PASSED IN**
        Console.WriteLine("comparing " + rhs.priority+"to "+this.priority); 

        if (rhs.priority <= this.priority)
        {
            Console.WriteLine("Entered first if");
            Console.WriteLine("comparing " + rhs.submitted + "to " + 
                this.submitted);
            if (rhs.submitted <= this.submitted)
            {
                Console.WriteLine("Entered second if");
                return 1;
            }
            else
            {
                return -1;
            }
        }
        else
        {
            Console.WriteLine("In else");
            return -1;
        }
    }
}
}

保存列表的类定义如下:

namespace PrioritisedRequestQueue
{
    public class Class1
    {
        public List<XactTask> tasks;

        public Class1()
        {
            tasks = new List<XactTask>();
        }

        public void sort()
        {
            tasks.Sort();
        }

        public void add(XactTask task)
        {
            tasks.Add(task);

        }

    }
}

这是我用来进行测试的程序:

PrioritisedRequestQueue.Class1 tester = new PrioritisedRequestQueue.Class1();

tester.add(new PrioritisedRequestQueue.XactTask(1, DateTime.Now, "test1"));
tester.add(new PrioritisedRequestQueue.XactTask(1, DateTime.Now, "test2"));
Console.WriteLine(tester.tasks[0].name);
Console.WriteLine(tester.tasks[1].name);
tester.tasks.Sort();

测试程序的输出是:

 comparing 1to 1 Entered first if
 comparing 16/05/2011 14:41:57to 16/05/2011 14:41:57 
 Entered second if
 comparing 1to 1 Entered first if
 comparing 16/05/2011 14:41:57to 16/05/2011 14:41:57 
 Entered second if
 comparing 1to 1 
 Entered first if
 comparing 16/05/2011 14:41:57to 16/05/2011 14:41:57 
 Entered second if
 comparing 1to 1 
 Entered first if
 comparing 16/05/2011 14:41:57to 16/05/2011 14:41:57 
 Entered second if

然后它似乎将一个nullrhs对象传递给compare方法,并NullReferenceException在尝试访问rhs的优先级时得到一个.

你能告诉我为什么会这样,我该怎么做才能解决这个问题？

如果您需要更多信息,请与我们联系.

Answer 1

当我运行你的代码时,我得到一个InvalidOperationException因为CompareTo没有正确实现.

如果相同,则您的CompareTo实现必须返回(即,此对象与被比较的对象相等).它也不应该失败,但应该以一致的方式排序; 这通常通过始终返回到所有有效对象之前的排序来完成.0rhsthisrhs == nullnull1null

IComparable.CompareTo的文档列出了以下规则,必须遵循这些规则才能使Sort正常运行:

• A.CompareTo(A) 需要返回零.

• 如果A.CompareTo(B)返回零,则 B.CompareTo(A)需要返回零.

• 如果A.CompareTo(B)返回零并 B.CompareTo(C)返回零,则 A.CompareTo(C)需要返回零.

• 如果A.CompareTo(B)返回非零值,则B.CompareTo(A) 需要返回相反符号的值.

• 如果A.CompareTo(B)返回值x不等于零,并 B.CompareTo(C)返回与x相同符号的值y,则 A.CompareTo(C)需要返回与x和y相同符号的值.

要解决此问题,请确保您的实现CompareTo返回0时priority == rhs.priority && submitted == rhs.submitted.